Making a two part mold: Casting resin models

I venture into creating silicone molds...

Having spent the better part of two months working on sculpting a true-scale Space Marine, it became clear that I could not continue to resculpt power armor each time that I wanted to make a new true-scale marine. I decided that the surest solution would be to make a rubber mold to cast additional copies of the armor. Although the process of making a silicone mold and using it to create resin casts is relatively straightforward, it was something that I had absolutely no knowledge about, so I was quite apprehensive about it all. After watching a YouTube video or two about the process, I decided I would give it a go, and ordered a starter kit from Smooth-On. Now that I have made my first mold, I wanted to share some of the things that I learned from the experience to hopefully encourage others to start creating molds to cast custom-made or sculpted components.

Instead of going straight to making a mold of the true-scale Space Marine, I decided it would be best to make a test mold with another model that I would be less devastated if things went awry. Therefore, I decided to start with something simple, and chose two relatively static, single piece metal models: a modified version of the original Slambo model and an old chaotic Space Ork from Rogue Trader. Both models were modified with greenstuff, with Slambo having resculpted boots, allowing me to see how the silicone reacted with greenstuff.

The two models that I decided to create a mold for. To make the process easier, each is a single piece metal model. They both have green stuff work done on them, allowing me to test how it reacts to the silicone mixture.

Creating a containment frame and preparing the mold
The first step of the mold making process is to create a containment frame around the model that you want to cast. This serves as a barrier to hold the liquid silicone/rubber around the model as it solidifies. I assembled my containment frame with Lego bricks and a Lego base plate. Lego bricks work well because they allow you to create larger and smaller enclosures with minimal effort. Since we are creating a two piece mold, you need a way to “mask” half of the model from the silicone, allowing you to create the first half, before unmasking it to create the second half. To do this, you fill the bottom of your Lego enclosure with modeling clay and push the model you are casting into the clay. When pouring the silicone, the half of the model embedded in the clay is not exposed to the liquid silicone. After the silicone solidifies, you can pull the model and silicone off of the clay (the solidified silicone being the first half of the mold). You can then pour new silicone onto the part of the model that was liberated from the clay, forming the second half of the mold. In addition to adding the models to the clay, you also need to add something to create the resin injection port/gate. I used pieces of Games Workshop sprue for this. To ensure that the two mold halves fit together precisely, I gouged notches at regular intervals into the clay (with a bottle of Krazy glue). These fill with with silicone and create “teeth” that hold both mold halves together and prevent them from shifting.

1. I laid out how I wanted with models in the mold within an enclosure made of Lego bricks, using pieces of GW sprue to make an injection port. 2. The models and sprue were embedded in modeling clay that was put in the bottom half of the containment frame.

Applying mold release is important

Before pouring the silicone rubber into the containment field, it is important you coat your model and the clay with a mold release. This prevents the model from getting stuck to the rubber after it cures. I used Ease Release 205 (Mann Release Technologies), which was provided in the Smooth-On starter kit. The solution was applied via a spray bottle. I used an old paintbrush to make sure all the recesses of the models were covered in the solution. After applying the solution I waited for a minimum of fifteen minutes for the solution to dry before proceeding. 

Pouring the silicone rubber

The Smooth-On starter kit that I used contains a two-part silicone rubber (Oomoo 30) for mold making and a two-part liquid plastic (Smooth-Cast 300) for casting. For the silicone rubber, the two parts (Part A, Part B) need to be mixed in equal parts by volume. Up to this point I have measured the two parts by volume in separate plastic disposable cups, followed by pouring the contents of one cup into the other for mixing. While this has worked, I have found that it a sizeable amount of the liquid remains stuck to the side of the cup while pouring, resulting in an uneven ratio of the two components. A better solution would be to use a digital scale for measuring the two components, taring the scale after measuring out the first part, allowing you to pour the second part into the same cup for mixing.    

After combining the two parts of the silicone rubber, you have a 30 minute working time before the rubber begins to cure and is difficult to work with (in practice it seems to be even shorter). When pouring the mixture into the enclosure, it is important to start the pour relatively high above it, allowing a thin stream of the silicone to flow into the enclosure. This helps pop any bubbles that might have formed in the mixing process. You should also make sure not to pour the silicone directly on top of the model, but instead let it flow naturally over the model. This will help prevent any regions of poor silicone coverage over your model. 

Creating the second half of the mold

After pouring the first half of the mold, it is suggested to wait around 6 hours before proceeding, to ensure it has completely cured. When it has, you break apart your Lego enclosure and separate the clay from the model and the silicone, taking care not to pull the model out of the newly solidified silicone. When this is done, you can rebuild the Lego frame around the first half of the silicone mold. Without the clay, there is now room for pouring new silicone to create the second half of the mold. You then repeat the mixing and pouring process from the first mold (remember to reapply the mold release solution!), and after it cures, the brunt of the two mold halves are done. For the molds to work efficiently, however, you need to make small modifications to these molds. Most of this can be done with an x-acto blade, cutting to ensure the injection port you created actually attaches to the model cavity, allowing resin to flow into the mold.

3. After the first half of the mold solidifies, the clay can be removed and a Lego frame can be rebuilt around the solidified silicone. With this done, the silicone can be poured again, forming the second half of the mold. 4. After both halves of the mold have solidified, you can remove the original models, and you have the basic mold. This needs to be modified to ensure it works effectively, cutting in additional vents.

Injecting the resin mixture into the mold

With the mold complete, the actual injection of resin is pretty simple. You mix the two-part liquid plastic (Smooth-Cast 300) in a 1:1 ratio and inject it via a plastic syringe into the injection port that you created in the mold. The resin solidifies within the span of 15 minutes, so you want to do the injection quickly. I used rubber bands to keep the two halves of the mold together tightly.

5. This is the result of my first cast with the mold. While the molds worked, there are many air bubbles in the resulting models (some indicated with the white arrows). To reduce the bubbles in the future, I need to modify the mold, adding additional vents.

Adding extra air vents is important to prevent air bubbles from forming in the model

One incredibly important element for ensuring the molds work efficiently is cutting vents into the mold at various places. These allow air bubbles to escape when injecting the resin, rather than getting trapped in the mold and ending up in the final resin model. This particular step involved a lot of trial and error experimentation, where I would use the mold to a cast the models and, based on where bubbles appeared, I would cut new vents into the mold. While none of my casts were perfect, with the careful addition of vents, I dramatically reduced the amount of air bubbles and places that did not fill with resin.

6. Using an x-acto knife, I cut in vents, primarily attaching to weapons and any area that had a lot of bubbles in the original cast (left). You can see that after resin is injected, it seeps out of the added vents rather than creating bubbles where air could not escape (right).

The original Slambo next to the resin cast.

Now that I have created my first mold and cast some models from it, I have a much deeper appreciation for the process. This is particularly true for creating one that minimized bubbles (something that GW struggled with with Finecast). Like all things, creating molds is a learning process. I think that I have learned enough that my next attempt should go more smoothly (my plan is to cast the True-scale Space Marine I have been working on). I hope this post was helpful to anyone considering starting to create their own molds, and would love to hear feedback or suggestions, based on any of your experience with the process!

-Adam Wier